Deployable propulsion for satellites

It took a large hangar to unfold the four ultra-lightweight booms, each made of carbon fibre-reinforced composites and 13.5 metres long, arranged in a cross shape. Researchers from the German Aerospace Center (Deutsches Zentrum fur Luft- und Raumfahrt; DLR) tested the booms twice in the aircraft hangar at the DLR site in Braunschweig.

In cooperation with the US space agency NASA, the aim is to develop deployable satellite structures that will make low-cost, small satellites more powerful in the future with deployable, miniaturised structures for power supply, communications and propulsion. The researchers will present the results of these tests at the 16th European Conference on Spacecraft Structures, Materials and Environmental Testing (ECSSMET), which will take place virtually for the first time this year from 23 to 25 March 2021.

The full-size booms under test are intended for a solar sail. Solar sails convert sunlight into thrust with the help of reflective membranes and can be used as propulsion for satellites. The planned sail size of 500 square metres will be able to generate a thrust of approximately 0.5 grams (for comparison, a single peanut weighs approximately 1.5 grams).

Even though this thrust is relatively small compared to other propulsion systems, it acts on the sails without interruption - with this constant 'propulsion', solar sails could not only be used in today's missions, but with the appropriate size could even enable mission durations that cannot be realised today. They not only have a long time window for their deployment, but also have another decisive advantage - they do not need a single gram of fuel for their propulsion.

Compact, lightweight and effective
With the 'Joint Deployable Space Structures / Deployable Composite Boom' (JDSS/DCB) cooperation project, DLR and NASA are taking advantage of the opportunities offered by increasingly available deployable structures as the miniaturisation of electronic components and sensor technology progresses. Until now, the available structural technologies could not keep up with miniaturisation and were designed for larger systems. Especially in terms of mass, stowage volume and stiffness requirements, conventional technologies quickly reach their limits when used on small satellites.

In this project, the DLR Institute of Composite Structures and Adaptive Systems is primarily responsible for developing a compact stowable and deployable structure for flat communications and radar antennas as well as solar arrays, while NASA is responsible for developing and manufacturing the carbon-fibre-reinforced composite booms. The technology was jointly tested during trials conducted at the DLR hangar in Braunschweig. The long-term goal of the cooperation is a mission in which the deployable structures that have been developed will prove their functionality under realistic conditions during tests in space.

Step by step to a space-qualified system
The successfully tested deployable booms have a closed cross-section formed by two omega-shaped half shells. Due to this special cross-section, such a boom has a high bending strength as well as a high torsional stiffness. As a result, it can take very high loads without buckling.

In addition, these booms allow the construction of a highly efficient 'backbone' for the tensioned solar sail. When rolled up on a spindle, the two half-shells are pressed together so that the boom becomes flat. For the test, a total of four 13.5-metre-long booms were rolled up and then unrolled in a cross shape so that they could resume their omega-shaped cross-section and thus regain their mechanical properties.

"We were able to successfully carry out this crucial process of rolling up and unrolling again in our test," says Project Manager Marco Straubel from the Braunschweig-based DLR Institute of Composite Structures and Adaptive Systems. "With this, we have confirmed that it is possible to unfold a large solar sail completely autonomously from a small container using the deployable booms."

As early as 2009, DLR conducted the first tests on deployable booms for satellites. DLR and NASA have been cooperating on the JDSS/DCB project since 2017. In the next step, the deployable construction will be tested on a DLR parabolic flight under microgravity conditions in summer 2021. After this, the system will be sufficiently qualified for the next logical step - testing in Earth orbit.

ECSSMET
The 16th European Conference on Spacecraft Structures, Materials and Environmental Testing (ECSSMET) will take place virtually for the first time this year from 23 to 25 March. Designers, analysts and test engineers working on space structures will not only discuss new subsystems this year, but also examine the topics that will play an important role in spaceflight in the near future - more efficient launchers, 3D printing in orbit and re-entry predictions for space debris. The conference is organised every two years, alternating between ESA, CNES and DLR.

Related Links
DLR Institute of Composite Structures and Adaptive Systems
Space Technology News - Applications and Research

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